The present invention relates to a novel form of precipitated aragonite and to a novel process for producing it.
Various routes are known for the production of calcium carbonate, which finds use as a thickening material, as a filler, as an extender, and most of all as a pigment, in a variety of industries such as pharmaceuticals, plastics, adhesives, printing, coating (paint), paper, rubber and in filtration. For such purposes, there may be used ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC). PCC in general possesses advantages over GCC, in that it is economical to produce and its precise composition, or purity, can be more strictly controlled.
The most frequently used chemical process for producing PCC is based on the carbonation of aqueous suspensions of calcium hydroxide (also known as xe2x80x9cmilk of limexe2x80x9d or xe2x80x9cslaked limexe2x80x9d) with carbon dioxide gas, or with a carbon dioxide containing gas. This process gives rise to relatively pure precipitated calcium carbonate and is a preferred process, because there are no serious problems of contamination of the product with undesired salts, and moreover it can be controlled in order to adjust the properties of the final product. Thus, the process is based essentially on four stages: firstly, calcination of raw limestone to produce calcium oxide or xe2x80x9cquicklimexe2x80x9d and carbon dioxide gas or a carbon dioxide containing gas; secondly, xe2x80x9cslakingxe2x80x9d of the quicklime with water to produce an aqueous suspension of calcium hydroxide; thirdly, carbonation of the calcium hydroxide with carbon dioxide gas or a carbon dioxide containing gas; and finally, downstream operations such as dewatering, drying, deagglomeration, grinding, surface treatment, surface coating, mixing with other minerals (e.g. titanium dioxide, talc, kaolin, GCC, PCCxe2x80x94including aragonite PCC) and dyeing, which allow optimization of the properties of the precipitated calcium carbonate particles in order to be adapted to their intended uses.
Calcium carbonate can be precipitated from aqueous calcium hydroxide slurries or solutions in three different crystallographic forms (polymorphs): the vaterite form which is thermodynamically unstable, the aragonite form which is metastable under normal ambient conditions of temperature and pressure, and the calcite form which is the most stable and the most abundant in nature. These forms of calcium carbonate can be prepared by carbonation of slaked lime by suitable variations of the process conditions.
The calcite form is easy to produce on industrial scales, as precipitated calcium carbonate particles. It exists in several different shapes, of which the most common are the rhombohedral shape and the scalenohedral shape.
Aragonite forms crystals having a length/width ratio (hereinafterxe2x80x94xe2x80x9caspect ratioxe2x80x9d) in the range between  greater than 1:1 and 100:1 of which a typical aspect ratio is 10, in which case the aragonite forms long, thin needles. Therefore, aragonite having a high aspect ratio may be denoted hereinafterxe2x80x94xe2x80x9cacicular aragonitexe2x80x9d or xe2x80x9cneedle-shaped aragonitexe2x80x9d. The production of aragonite is a slow process and is very difficult to control on an industrial scale.
PCC particles are used as thickening materials, fillers, extenders and, most of all, as inexpensive pigments. The latter use implies that a particularly desirable property of this material is its light scattering characteristics, in order to be able impart opacity and brightness to the products containing it. Such characteristics are optimized, when the pigment particles are very effectively dispersed and are apart by an average distance in the range between 0.2 xcexcm and 0.4 xcexcm in their final products, and their size distribution is in the range between 0.2 xcexcm and 0.4 xcexcm, namely, in the range of half a wavelength of the visible light. That means that either the production of the PCC should be adjusted to produce small particles in order to avoid expensive downstream particle size reduction operations and to cope with the expensive problems of dewatering and drying the product, or, alternatively, the process should be adjusted to produce large particles, and subsequently effect the downstream dewatering and grinding operations. In both cases, the production costs of precipitated calcium carbonate of pigment grades may be doubled or tripled just because of these unavoidable downstream steps.
High light scattering pigments currently available to the above-mentioned industries include titanium dioxide particles, which are very effective to scatter the light due to their relatively high refractive index (2.76; for the rutile form) and their meticulously controlled particle size distribution of which median is in the range between 0.2 xcexcm and 0.4 xcexcm. However, this product is of a high specific gravity (xcx9c4.0 g/cm3), of a high surface area due to its small particles, and most of all, is quite expensive. Fine kaolin particles are also being used as pigments, but this product has a much lower refractive index (1.56), is of limited whiteness and is still relatively expensive. Particulate calcium carbonate is the ideal least expensive pigment and could replace much more of the titanium dioxide and kaolin pigments in their respective present applications, if it could be prepared in a form having improved light scattering properties.
Calcium carbonate pigments are produced in part by grinding coarse natural rocks and in part by precipitation processes. Of the precipitated calcium carbonate particles, a particulate precipitated aragonite is considered to be the most effective light scattering calcium carbonate pigment, of which refractive indices are 1.530, 1.681 and 1.685, depending on its crystallographic surfaces, its specific gravity is above 2.5 g/cm3, and is the most suitable for same applications. However, its production rate is characteristically very slow and its production conditions are very difficult to control, industrially.
While the majority of references, cited hereinafter, relate to the technology for producing a particulate precipitated aragonite, some of the references are included in order to better present the state of the art for the production of PCC more generally, including the downstream operations, which may be common to all these processes and also to the present invention.
1. U.S. Pat. No. 2,081,112 (N. Statham et al.) describes a process for producing precipitated calcium carbonate by carbonating milk of lime with carbon dioxide containing gas, where the temperature in the gas absorber is maintained at 50-60xc2x0 C., preferably around 55xc2x0 C. It is recognized that the more violent the agitation in the gas absorber, the finer will be the product; the aim being to create a fine mist of calcium hydroxide slurry.
2 U.S. Pat. No. 2,964,382 (G. E. Hall, Jr.) describes production of. precipitated calcium carbonate by various chemical routes, in which calcium ions are contacted with carbonate ions in a precipitation zone, the process including also carbonation of milk of lime with carbon dioxide gas. A high shear stator/rotor agitator is used to provide turbulence by rotating at a peripheral speed of at least 1160 feet per minute (589 cm per second) in the precipitation zone. Also, this patent teaches that it is desirable to operate the process at pH values of at least 8.5 and that at temperatures above 60xc2x0 C., needle-shaped precipitated aragonite particles are formed, which however produce an adverse flow property effect.
3. U.S. Pat. No. 3,320,026 (W. F. Waldeck) describes the production of various forms of precipitated calcium carbonate.
4. GB Patent No. 941,900 (assigned to Kaiser Aluminium and Chemical corporation) describes the production of precipitated aragonite particles, for use as a filter aid, by reacting continuously sodium carbonate solution and aqueous calcium hydroxide slurry at temperatures higher than 60xc2x0 C. in a multistage system. The product and the solution are withdrawn at the third stage from the bottom of the reactor, the product is then separated from the solution and part of the crystals are recycled to the various stages of the process as seeds for further precipitation of the precipitated aragonite particles.
5. U.S. Pat. No. 3,669,620 (M. C. Bennett et al.) describes a continuous process for the production of a particulate precipitated aragonite by carbonating aqueous calcium hydroxide slurry in sucrose solutions. However, due to the cost of the sucrose, the solution had to be recycled and detrimental materials had to be removed by anion exchange resin. The preferred temperature range was between 60xc2x0 C. and 90xc2x0 C.; the pH values were in the range between 7 and 9; and the concentration of the calcium hydroxide was quite lowxe2x80x94in the range between one-half and one-twentieth molar.
6. U.S. Pat. No. 4,018,877 (R. D. A. Woode) describes carbonation of calcium hydroxide slurry wherein a complexing agent for Ca++ is added to the suspension in the gas absorber, after the calcium carbonate primary nucleation stage and before completion of the carbonation step, the complexing agent being e.g. citric acid, ethylenediamine tetraacetic acid (EDTA), aminotriacetic acid, aminodiacetic acid or a hydroxy polycarboxylic acid. Optionally, long-chain fatty acids or their salts can be added, preferably, after the final carbonation stage.
7. U.S. Pat. No. 4,157,379 (J. Arika et al.) describes the production of a chain-structured precipitated calcium carbonate by the carbonation of calcium hydroxide suspended in water in the presence of chelating agents, such as aliphatic carboxylic acids, and water-soluble metal salts.
8. U.S. Pat. No. 4,244,933 (H. Shibazaki et al.) describes a multi-stage production process for producing a particulate precipitated aragonite, using aqueous calcium hydroxide slurry and carbon dioxide gas or a carbon dioxide containing gas, in the presence of phosphoric acids and water-soluble salts thereof.
9. U.S. Pat. No. 4,420,341(T. H. Feringo) describes inorganic fillers (including calcium carbonate) surface modified with carboxylic acids, antioxidants and high-boiling non-reactive liquid agents.
10. JP Patent Publication No. 63260815 (H. Shibata et al.) describes the production of a particulate precipitated aragonite, by reacting carbon dioxide gas with an aqueous calcium hydroxide slurry in presence of phosphoric acid, a phosphoric acid compound, a barium compound and a strontium compound.
11. JP Patent No. 1261225 (H. Shibata et al.) describes reacting carbon dioxide gas with an aqueous calcium hydroxide slurry, in order to produce a particulate precipitated aragonite, which is stated to have improved properties compared with particulate precipitated calcite.
12. U.S. Pat. No. 4,824,654 (Y. Ota et al.) describes a process for producing precipitated needle-shaped (5-100 xcexcm) particulate precipitated aragonite, in which a relatively dilute aqueous calcium hydroxide solution (0.04-0.17 wt. %) and carbon dioxide gas or a carbon dioxide-containing gas are reacted together at a temperature of not less than 60xc2x0 C., in a continuous or semi-continuous (intermittent) manner. [That can be said about the entire relevant prior art]
13. U.S. Pat. No. 5,043,017 (J. D. Passaratti) describes a process for producing acid-stabilized precipitated calcium carbonate particles.
14. U.S. Pat. No. 5,164,172 (H. Katayama et al.) describes a process for producing a particulate precipitated aragonite, in which a mixture of aqueous calcium hydroxide slurry, aragonite calcium carbonate particles and a water-soluble phosphoric acid compound are premixed prior to the addition of carbon dioxide gas.
15. U.S. Pat. No. 5,342,600 (I. S. Bleakley et al.) describe a process of producing particulate precipitated calcium carbonate, in which aqueous calcium hydroxide slurries of varying concentrations are reacted with carbon dioxide-containing gas under a controlled mixing speed. It is recommended therein to prepare the aqueous calcium hydroxide suspension under high shear mixing and subsequently to lower the energy and shear agitation in the reaction mixture in which the precipitated calcium carbonate particles are formed.
16. U.S. Pat. No. 5,376,343 (P. M. Fouche) describes a process for producing various forms of particulate PCC. In the case of aragonite, a mixture of quite dilute aqueous calcium hydroxide solution and a water-soluble source of specific anions (e.g. ammonium nitrate) are premixed prior to addition of CO2 gas.
17. U.S. Pat. No. 5,380,361 (R. A. Gill) describes inter alia calcium carbonate particles coated with C12-C22 fatty acids salts.
18. U.S. Pat. No. 5,593,489 (K-T. Wu) describes a process for producing acid-resistant calcium carbonate particles for making neutral to weakly acid paper.
19. U.S. Pat. No. 5,833,747 (I. S. Bleakley et al.) describes a process for producing a particulate precipitated aragonite, in which an aqueous calcium hydroxide slurry (148 g Ca(OH)2 per liter of suspension) is reacted with carbon dioxide gas at an exceptionally slow rate of 0.0026 moles per minute per mole of Ca(OH)2 in a batch operation.
20. WO 9852870 (B. Jackson et al.) describes a multi-stage commercial process for producing a particulate precipitated aragonite, using coarse-grained precipitated aragonite particles as a seeding material. Though the process is claimed to be industrially applicable, it is quite slow and thus of very limited economical value.
21. U.S. Pat. No. 5,846,500 (J. W. Bunger et al.) describes a process for producing a particulate precipitated aragonite, in which an aqueous calcium hydroxide solution is reacted with CO2 gas in a plug-flow reaction system.
22. U.S. Pat. No. 5,846,382 (A. von Raven) describes a process for producing inorganic fillers and pigments, including particulate calcium carbonate, of improved whiteness, brightness and chromaticity.
23. U.S. Pat. No. 5,861,209 (W. J. Haskins et al.) describes a process for producing a particulate precipitated aragonite, for printing, in which an aqueous calcium hydroxide slurry is first mixed with precipitated aragonite particles for seeding and then it is reacted quite slowly with carbon dioxide gas in a batch operation. After dewatering the product to a cake containing about 70% solids, it is mixed with a typical dispersant, e.g. sodium polyacrylate, and it is further dispersed. This patent discloses the use of mixtures of a particulate precipitated aragonite, with TiO2 and other inorganic fillers, pigments and flame retardants.
24. U.S. Pat. No. 5,939,036 (A. L. Porter et al.) describes a process for producing a particulate precipitated aragonite, in which aqueous mixtures of organic compounds and acids (e.g. ethanclamine and HCl) are used to dissolve impure CaO and to form a calcium hydroxide mixture, which is then reacted with carbon dioxide gas to yield various forms of PCC, depending on the temperature. Controlling the temperature of the carbonation at about 95xc2x0 C. leads to aragonite.
25. U.S. Pat. No. 6,022,517 (G. H. Fairchild et al.) describes a process for producing mixtures of precipitated acicular calcite and acicular aragonite particles in the ratio of 75:25 to 25:75, by reacting carbon dioxide gas or a carbon dioxide containing gas and aqueous calcium hydroxide in the presence of a water soluble aluminum compound, by controlling the specific conductivity in a range  greater than 4.0 and up to about 7.0, milliSiemens/cm, at a reaction temperature of from 25-60xc2x0 C.
26. Pigment Handbook (Vol. I-III; Edited by T. C. Patton; John Wiley and Sons, New York (1973)) describes the properties, the production processes and various uses of aragonite calcium carbonate pigment (c.f. Vol. I; Pages 119-128), as well as those of other pigments that compete in the same market like titanium dioxide, kaolin, GCC, etc. The discussion concerning the influence of the film porosity on the hiding power or opacity of a coating film (c. f. Vol. III; Pages 203-217 and especially on Page 212) may help in understanding some aspects of the present invention.
The entire contents of the above-cited literature, including patents and patent publications, are incorporated herein by reference. It is apparent from the state of the art that known processes for the industrial production of substantially pure particulate precipitated aragonite ( greater than 90 parts aragonite:  less than 10 parts calcite), by reacting aqueous calcium hydroxide slurries with carbon dioxide gas or a carbon dioxide containing gas, exhibit serious drawbacks that affect the quality and cost of the final product, as follows:
A. Some of the processes are conducted in aqueous solutions of extremely low concentrations of calcium hydroxide. In some cases it is specified that clear solutions, which contain less than 1 wt. % calcium hydroxide, should be used.
B. In those processes which allow use of aqueous calcium hydroxide slurries, the production rates are very slow and difficult to control.
C. To increase somewhat the rates of production in processes of A and B, the prior art recommends seeding with previously produced aragonite particles. However, this complicates the production processes, especially those operated continuously, and which are otherwise of great commercial potential.
D. Dewatering of particulate precipitated aragonite obtained according to the known art gives rise to relatively wet filter cakes of which the water content is not below 30% and which may thus require a very expensive subsequent drying step.
E. Particulate precipitated aragonite of the prior art requires extensive grinding operations to optimize its particle size distribution (PSD) in order to meet the effective PSD in the range between 0.2 xcexcm and 0.4 xcexcm, mentioned above. Moreover, the grinding operation tends to contaminate the product, due to attrition of the grinding media, unless very expensive materials of construction are used for this purpose.
F. The known particulate precipitated aragonite is of limited whiteness, mainly due to the high residual impurities in the CaCO3/CaO feedstock, which it is quite difficult to remove thoroughly, on the industrial scale. Also, the low whiteness of the product is a limiting factor in choosing the suitable sources of its raw materials (CaCO3/CaO).
G. Particulate precipitated aragonite frequently requires one or more post-manufacturing treatment step(s), in order to ensure that the particle surface is hydrophobic, by coating with suitable long-chain carboxylic acids and/or other materials such as silicon greases, e.g. for efficient dispersal in hydrophobic media such as rubber or plastics, and/or to ensure resistance to acidic environments for use e.g. in the paper industry and in the coating industry.
H. Efforts in the prior art to increase the effective refractive index of particulate precipitated aragonite has not so far succeeded in making this material a serious competitor to titanium dioxide.
Accordingly, it is an object of the present invention to overcome all or most of the problems encountered in:the prior art, as mentioned in paragraphs A-H, above.
It is an object of the present invention to provide particulate precipitated aragonite, as stated in the preceding paragraph, by a process which is more efficient and less expensive, than those available in the prior art.
It is yet a further object of the present invention to effect such a more efficient and less expensive process as stated in the preceding paragraph, using sources of CaCO3/CaO, which are presently not suitable raw materials for use as e.g. fillers, extenders and pigments, and for other applications, in all of which uses require pigments of high optical properties and high performance, whereby production costs are lowered.
Still another object of the present invention is to provide a particulate precipitated aragonite of a superior quality as stated above, in which the produced particles are treated in situ with a hydrophobic agent in order to avoid an extra downstream step and to fine-tune their properties to meet the requirements of the rubber, plastics, coatings (especially durable paints in acidic environments), inks and paper industries (especially paper production in weakly acidic media), an effect of said in situ treatment being lowering of production costs.
Still another object of the present invention is to carry out the above-stated more efficient and less expensive process, in a manner which gives rise to filter cakes which are relatively dry, e.g. with no more than about 20 wt. % water, right after the dewatering stage, and thus additionally lowering production costs.
Another object of the invention is to effect the above-stated more efficient and less expensive process, in such a manner that the produced particulate precipitated aragonite does not require, for most applications, any downstream grinding operations, except for the regular mixing systems which are in any event usually installed in the industries mentioned above, and thus additionally lowering production costs.
Most of all, it is a particular object of the present invention to provide a particulate precipitated aragonite of a better quality than that obtained in the prior art, and especially having a higher whiteness, a lower specific gravity and a higher effective refractive index.
Other objects of the invention will appear from the description which follows.
It has been surprisingly found in accordance with the present invention, that an industrially viable particulate precipitated aragonite calcium carbonate, which is characterized by its high whiteness, high effective refractive index, and especially by its low specific gravity (below 2.5 g/cm3), can be produced, and that the above-mentioned objects of the present invention can be achieved, by a process which comprises reacting an aqueous calcium hydroxide slurry with a gas selected from carbon dioxide and a gas containing it, wherein the parameters of the process, including e.g. at least one preselected active agent, modes of operation, operating concentrations of raw materials, operating temperatures, operating pH range and high shear mixing speeds are strictly controlled such that the desired product is obtained. In a particular embodiment, flotation of the product occurs during such process
The process of the invention for producing a particulate precipitated aragonite in accordance with the invention, is preferably further characterized by at least one, and preferably all, of the following features: (a) the active agent comprises at least one member selected from the group consisting of carboxylic acids of formula CH3(CH2)nCOOH, where n is 7-9, and their carboxylate salts, esters, anhydrides, and acyl halides, and ketenes of formula CH3(CH2)nxe2x88x921Cxe2x95x90Cxe2x95x90O; (b) the concentration of the active agent is within the range of between 0.2 wt. % and 10 wt. %, calculated as CH3(CH2)nCOOH and based on the weight of calcium carbonate, (c) the slurry contains calcium hydroxide in a concentration within the range of from 3 to 30 wt. %, more preferably 4 to 20 wt. %; (d) the product is produced at a pH between 8 and 11, preferably between 9 and 10; (e) the process is effected at a temperature in the range between 60xc2x0 C., desirably between 80xc2x0 C., and the boiling temperature of the reaction mixture; (f) the process is effected either in a semi-continuous (intermittent) mode of operation, or more preferably in a continuous manner; (g) the process is effected under high shear mixing e.g. with a mixer comprising a rotor/stator or a rotor only, the mixer peripheral (tip) speed being preferably at least 5 m/sec. In a particular embodiment, this process is effected in a continuous mode of operation under high shear mixing with a mixer comprising a rotor/stator or a rotor only, at a temperature in the range between 90xc2x0 C. and the boiling temperature of the reaction mixture, the active agentxe2x80x94preferably present in an amount in the range between 0.2% and 10 wt. %, calculated on the weight Of calcium carbonatexe2x80x94being selected from the Carboxylic acids and their calcium salts, and the slurry contains calcium hydroxide in a concentration within the range of from 5 to 15 wt. %, the active agent being desirably premixed with the calcium hydroxide slurry prior to reaction with carbon dioxide. The present invention also provides as a novel chemical substancexe2x80x94which is of course obtainable in accordance with the present process, a particulate precipitated aragonite, which has a specific gravity of  less than 2.5 g/cm3 (preferably  less than 2.3 g/cm3, more preferably  less than 2.0 g/cm3, even more preferably  less than 1.8 g/cm3) after drying at 120xc2x0 C., and a specific gravity  less than 2.5 g/cm3 after ignition for eight hours at 500xc2x0 C. In a particular embodiment, the product has a specific gravity of  less than 2.3 g/cm3 after drying at 120xc2x0 C., and a specific gravity  less than 2.3 g/cm3 after ignition for eight hours at 500xc2x0 C.
A typical such product may be further characterized by at least one of the following features; it contains said carboxylate calcium salt(s) in an amount between 0.2 and 10 wt. %, calculated as CH3(CH2)nCOOH and based on the weight of calcium carbonate; it has a specific gravity  less than 2.2 g/cm3, preferably  less than 2.0 g/cm3, more preferably  less than 1.8 g/cm3; a product previously dried at 120xc2x0 C. for 12 hours has a loss on drying at 300xc2x0 C. for 8 hours of  less than 10% wt %, based on the weight of calcium carbonate; a product previously dried at 120xc2x0 C. for 12 hours has a loss on ignition at 500xc2x0 C. for 8 hours of  less than 10% wt. %, based on the weight of calcium carbonate; after drying at 120xc2x0 C. for 12 hours, and/or drying for 8 hours at 300xc2x0 C., and/or firing for 8 hours at 500xc2x0 C., it still has a specific gravity  less than 2.5 g/cm3.
The product of the present invention can be used as a thickening material, a filler, an extender and particularly as a pigment for the pharmaceuticals, plastics, adhesives, printing, coating (paint), paper, rubber, filtration, and many other industries.